1. Field of the Invention
The present invention relates to a ceramic chip inductor and a method for producing the same, and in particular, a lamination ceramic chip inductor used in a high density circuit and a method for producing the same.
2. Description of the Related Art
Recently, lamination ceramic chip inductors are widely used in high density mounting circuits, which have been demanded by size reduction of digital devices such as devices for reducing noise.
As an example of the conventional art, a method for producing a conventional lamination ceramic chip inductor described in Japanese Laid-Open Utility Model Publication No. 59-145009 will be described.
On each of a plurality of magnetic greensheets, a conductive pattern formed of a conductive paste of less than one turn is printed. The plurality of magnetic greensheets are laminated and attached by pressure to form a lamination body. The conductive lines on the magnetic greensheets are electrically connected with each other sequentially via a through-hole formed in the magnetic sheets to form a conductive coil. The lamination body is sintered entirely to produce a lamination ceramic chip inductor.
Such a lamination ceramic chip inductor requires a larger number of turns of the conductive coil and thus a larger number of greensheets in order to have a higher impedance or inductance.
An increase in the number of greensheets requires a larger number of lamination steps and thus raises production cost. In addition, such an increase raises the number of the points of connection between the conductive patterns on the greensheets, thus reducing the reliability of connection.
A solution to these problems is proposed in Japanese Laid-Open Patent Publication No. 4-93006. A lamination ceramic chip inductor disclosed in this publication is produced in the following manner.
On each of a plurality of magnetic sheets, a conductive pattern of more than one turn is formed using a thick film printing technology, and the plurality of magnetic sheets are laminated. The conductive patterns on the magnetic sheets are electrically connected to each other sequentially via a through-hole formed in advance in the magnetic sheets. A lamination ceramic chip inductor produced in this manner has a relatively large impedance even if the number of the magnetic sheets is relatively small.
Such a lamination ceramic chip inductor produced using a thick film technology has the following two disadvantages.
(1) In the production of a lamination ceramic chip inductor having an outer profile as small as, for example, 2.0 mm.times.1.25 mm or 1.6 mm.times.0.8 mm using a thick film printing technology, the number of turns of each conductive pattern is approximately 1.5 at the maximum for practical use with the production yield and the like considered. In order to produce an inductor having a larger impedance, the number of the magnetic sheets needs to be increased.
(2) In order to increase the number of turns in one magnetic sheet, the width of each conductive pattern needs to be reduced. Since a reduced width of the conductive pattern increases the resistance thereof, the thickness of the conductive pattern needs to be increased. However, in order to maintain the printing resolution, the thickness of the conductive pattern needs to be reduced as the width thereof is decreased. For example, when the width is 75 .mu.m, an appropriate thickness of the conductive pattern when being dry is approximately 15 .mu.m at the maximum.
From the above description, it is appreciated that increasing the number of turns of each conductive pattern is not practical although effective to some extent in reducing the number of the magnetic sheets.
In order to reduce the resistance of the conductive pattern, Japanese Laid-Open Patent Publication No. 3-219605 discloses a method by which a greensheet is grooved, and the groove is filled with a conductive paste to increase the thickness of the conductive pattern. However, it is difficult to mass-produce a grooved greensheet in a complicated pattern.
Japanese Laid-Open Patent Publication No. 60-176208 also discloses a method for reducing the resistance of the conductive pattern of a lamination body having magnetic layers and conductive patterns each of approximately a half turn alternately laminated. In this method, the conductive patterns to be formed into a conductive coil are formed by punching a metal foil. However, it is difficult to punch out a pattern with sufficient precision to fit into a microscopic planar area as demanded by the recent size reduction of various devices. In fact, it is impossible to obtain a complicated coil pattern having one or more turns by punching. Further, it is difficult to arrange a plurality of metal foils obtained by punching on a magnetic sheet at a constant pitch with high precision. Moreover, when the metal foils adjacent to each other are connected with a magnetic sheet interposed therebetween, defective connection can undesirably occur unless the connection technology is sufficiently high.
A solution to the above-described problems from a different point view is disclosed in Japanese Laid-Open Patent Publication No. 64-42809 and Japanese Laid-Open Patent Publication 4-314876. In these publications, a metal thin layer formed on a film is transferred onto a ceramic greensheet to produce a lamination ceramic capacitor.
In detail, on a releasable metal thin layer formed on a film by evaporation, a desired metal layer is formed by wet plating. When necessary, an extra portion of the metal layer is removed by etching. The resultant pattern is transferred onto a ceramic greensheet.
Such a transfer method can be applied to transfer a conductive coil onto a magnetic greensheet in the following manner to produce a lamination ceramic chip inductor.
A relatively thin metal layer (having a thickness of, for example, 10 .mu.m or less) formed on a film is etched using a photoresist to form a fine conductive coil pattern (having a width of, for example, 40 .mu.m and a space between lines of, for example, 40 .mu.m). The resultant coil is then transferred onto a magnetic greensheet. In this manner, a lamination ceramic chip inductor for having a large impedance can be produced.
By the above-described transfer method, it is difficult to produce a relatively thick conductive coil having a pattern to be transferred (having a thickness of, for example, 10 .mu.m or more) for the following reason.
By the transfer method using wet plating, the metal layer which is once formed on the entire surface of a film is patterned by removing an unnecessary portion. Accordingly, production of a complicated coil pattern becomes more difficult as the thickness of the metal film increases.
Further, since the desired pattern is obtained under the photoresist, the photoresist needs to be removed before the transfer. When the photoresist is removed, the conductive coil pattern may also be undesirably removed. Such a phenomenon becomes easier to occur as the thickness of the metal layer increases. The reason is that: as the thickness of the metal layer increases, etching takes a longer period of time and thus the thin metal film is exposed to the etchant to a higher degree.
For the above-described reasons, the transfer method cannot provide a lamination ceramic chip inductor having a low resistance.